Thyroid peroxidase is one of the main enzymatic constituents of the thyroid together with thyroglobulin. It is involved in thyroid hormone synthesis. These proteins are coupled to the thyroid stimulating hormone receptor. Thyroid hormone synthesis is under complicated multi-hormonal regulation to which these three proteins are subjected. We have focused on studying human thyroid peroxidase in terms of gene regulation and biochemical characterization of the protein in order to understand the mechanism of thyroid hormone synthesis. We have identified a thyroid- specific enhancer-binding sequence located approximately 5.5 kbp upstream of the gene's transcription start site and have used this binding sequence to isolate a cDNA clone encoding the thyroid-specific enhancer- binding protein (T/EBP). T/EBP possesses the exact same sequence as thyroid-specific transcription factor-1 (TTF-1), which has been characterized as a homeodomain-containing DNA-binding protein. This protein interacts with the rat thyroglobulin gene promoter and confers thyroid specificity to rat thyroglobulin gene expression. In order to determine if T/EBP also functions as a transcription factor in the expression of the human thyroid peroxidase gene, we further examined the gene's promoter region and identified two and one T/EBP binding sites, respectively, in the proximal (within -153 bp) and the distal (about - 1000 bp) promoter regions. Loss of T/EBP's binding activity by mutating these binding sites was shown to be correlated with the loss of expression of the gene. We have also begun to establish a T/EBP-knockout mouse line which lacks the expression of the T/EBP gene. It should be of interest to determine the outcome of this study since T/EBP appears to be essential for the expression of the thyroid-specific genes and has been reported to be involved in the development of thyroid, lung, and brain.